Shaded pole induction motor



Feb- 16, 1937- E. w. BALLENTINE 2,071,224

SHADED POLE INDUCTION MOTOR Filed July 29, 1935 ILP Patented Feb. 1s, 1931 PATENT OFFICE SHADED POLE INDUCTION MOTOR Earle W. Ballentine, Chicago, Ill., assiguor to Solar Industries, Inc., Chicago, Ill., a corporation of Illinois Application July 29, 1935, Serial No. 33,752

3 Claims. (Cl. 172-278) This invention relates to improvements in electric motors. For the purpose of illustrating the invention, I have here shown the same embodied in a four-pole alternating current induction 5 motor.

' Among the features of this invention is the provision of a shading coil motor having a fourpole closed slot stator with coil slots adapted to contain two eld coils on opposite poles of said stator, in which said coil slots are symmetrically arranged with respect to all four poles.

Another 'feature of this invention is the provision of a four-pole two-coil motor using shading coils in which adjacent poles are integrally joined l5 with each other .to form an integral magnetic bridge between poles.

Yet another feature of my invention is the proportioning of the narrowest portions of the integral magnetic bridge and the proportioning of the size of the shaded portion vof each pole with respect to the unshaded portion to achieve a starting torque, pull out torque, and efIiciency hitherto unobtained in such motors.

Other features and advantages of this invention will appear from the following specification and the drawing, in which- Fig. 1 is a top plan view of such a motor; Fig. 2 is a view, partly in section, along the line 2-2 of Fig. 1; Fig. 3 is a fragmentary view along the 30 line 3-4 of Fig. 2; and Fig. 4, a fragmentary view along the line 4-4 of Fig; 2.

In the particular embodiment of the invention illustrated herewith, the stator comprises two sets of plates or laminations, the outer set being 35 indicated by the reference numeral': III and the inner set by the reference numeral I I.' The inner plates are shaped somewhat analogously to a cross, as may be seen in Fig. 2. The outer plates or laminations III are substantially in the shape of 40 an open square, with recessed portions corresponding to the four extending pieces I3 on the inner plates. The inner and outer plates are so shaped that openings I4, I5, I6, and I1 are adapted to receive the field coils I8 and I3. Each of 45 the inner plates I I has an opening 2II through the center thereof, this opening being adapted to accommodate the rotor 2l. The openings I4, I5, I3, and I1 are separated from the rotor opening 20 by the integral bridges 22, 23, 24, and 26,

60 respectively. Each pole 26 has a shaded portion 21, the shaded portion being separated from the ,remainder of the pole by a small slot 23. The shaded portion of the pole is surrounded by a shading coil 29, as is more clearly shown in Fig.

553. Theshadingcoilnisastripofcopperor other metal of high conductivity passing completely around the shaded portion of the pole, lying in the slot 2l on one side thereof and in a groove on the other side thereof. The copperstrip is fastened into a continuous ring by brazing, soldering or the like, at some point, as at 3|. 'I'here is thus a circular or continuous ring oi' magnetic metal entirely around the rotor without any gaps or slots except the small slots 23 which are adapted to receive the shading coils.

The rotor is composed of a series of stacked magnetic rotor laminations 32, conductor bars 33, and end rings 34 of copper or other material of high conductivity. The conductor bars 33 extend through openings punched in the magnetic laminations 32, and are riveted, as at 35, into the end` rings 34. 'I'he rotor is axially mounted on a shaft 33, which shaft is journalled in bearings 31 and 38 carried by transverse members 33 and 40. 'Ihese transverse membersArest-directly on the stator laminations, and are here shown as fastened thereto by the bolts 4I and 42.

f The assembly of the motor may be outlined as follows. The inner and outer stator laminations are stamped out in the shape illustrated, the inner laminations II are alined and fastened together, as by the rivets 43, and the outer stator laminations are likewise alined and fastened together, as vby the rivets 44. The shading coils may then be assembled in their proper position around the shaded portion of each pole. The rotor is then placed in the rotor opening 23 in the inner laminations, and the eld coils I3 and I3, previously form-wound and tape-insulated, are then placed over a pair of opposite poles on the inner stator laminations. The inner laminations, the rotor, and the coils may then be placedv in position over the central opening in the outer laminations, and slipped down into proper position therein. The laminations are preferably stamped so that the resultant llt is a tight one, thus insuring little or no increased reluctance to the magnetic flux at the junction points between the inner and outer laminations. It will be noted that the bolts 4I and 42 not only hold the transverse members 39 and 40 in position, but also positively lock the inner and outer laminations together, as may be seen in Fig. 2.

It has been found that in a four-pole induction motor of the shaded pole type described and illustrated heretofore, the proportion of the total pole ux passing through the shaded portion oi' the pole and the size of the narrowest portion of the integral magnetic bridge are highly important elements in the perfomance and efilciency of the motor. In a motor of this type the torque is a direct function of the shading coil inductance and the rotor resistance, and an inverse function of the rotor inductance and shading coil resistance. In order to have high starting and pull out torque it is thus important to have as large shading coil inductance and as low shading coil resistance as losses permit. The use of the integral magnetic bridges 22, 23, 24, and 25 increases the shading coil inductance through providing a shunt magnetic path with very low reluctance. The size of these bridges, however, must be carefully proportioned, since if they are too broad substantially all of the flux passes through the bridge and little or none passes through the rotor. It is only that portion of the ux passing through the rotor, of course, which provides driving force therefor, The cross-sectional area of the integral magnetic bridge must thus be proportioned to raise the inductance of the shaded pole, and yet not to divert such a large portion of the flux as to substantially decrease the amount thereof passing through the rotor. It has also been found that the proportion of flux passing through the shaded pole, and the `relative circular lag thereof, are important. A pole designed, as here illustrated, such that the cross-sectional area of the shaded portion of the pole is from 10 to 40% of that of the entire pole gives good results. The preferred area is such that substantially 21% of the total flux of each pole passes through the shaded portion thereof, and the lag of that flux is from 25 to 30 degrees with respect to the flux passing through the unshaded portion of the pole.

Flux density and reluctance are functions of the cross-sectional area of the magnetic circuit at any particular point. In the particular motor described and illustrated herein an equal number of laminations are used throughout, and the cross-sectional areas of the magnetic circuit at any two given points, therefore, have the same ratio as the widths of a single lamination at those points. These ratios will hereafter be described as the ratio of the Widths of a lamination at two given points. It is to be understood, however, that when the width of the integral magnetic bridge at the narrowest point is described as being from 20 to 30% of the width of the shaded portion of the pole it is meant that the cross-sectional areas of the flux paths at those two points bear that ratio.

It has been found that the width of the narrowest portion of the integral magnetic bridge is a function both of the rotor diameter and of the width of the shaded portion of the pole. The highest torque and the best efliciency combination is obtained when the width of this bridge is from 11/2 to 5% of the rotor diameter, and from 10 to.

to of the diameter of the rotor, being here i1- lustrated at its preferred value of 66%.

The particular motor heretofore described and illustrated gives a combination of torque and eiliciency never before attained in a motor of the shaded pole type. A motor of these proportions, with. a 21A inch rotor, develops a rated full load speed of 1700 R. P. M.. a speed higher than that heretofore achieved under full load by motors of this type. The starting torque is about of the rated full load torque, and the pull out torque is about 250% of the rated full load torque. The speed regulation under full load is very good, since the torque curve at4 1700 R. P. M. is rather steep. The torque curve, from its, starting point to its pull out point, is substantially a straight line curve, without the decrease in torque usually found at a point substantially corresponding with the third harmonic of the current supplied to the motor. Despite the high starting and acceleration torque, starting current is only about twice running current, and there is thus no danger of a burned out motor if it is unable to start the load. The motor hitherto described and illustrated as an embodiment of this invention, even though it provides high starting and accelerating torque, has a very good over-all electrical eiliciency. The peak eiiciency of this motor, at a speed just under the rated f-ull load speed, is from 30 to 35%, an efliciency equal to that of a resistance split-phase type of motor of a similar rotor diameter.

While I have shown and described certain embodiments of my invention, it is to be understood that it is capable of many modications. Changes, therefore, in the construction and ar rangement may be made without departing from the spirit and scope of the invention as disclosed in the appended claims, in which it is my intention to claim all novelty inherent in my invention as broadly as possible in view of the prior art. 'l .i i e What I regard as new, and desire to secure by Letters Patent, is:

1. An induction motor of the shaded pole type, having at least four poles, including: a rotor; a stator having poles of a width less than 75% of the diameter of said rotor, an integral magnetic bridge the narrowest portion of which is from 11/2 to 5% of the diameter of said rotor and a shaded portion of each pole of a. width of from 20 to 30% of the width of the entire pole; and a iield coil.

2. A four pole single phase induction motor of the shaded pole type, including: a rotor; a stator having poles of a width from 60% to 75% of the diameter of said rotor, an integral magnetic bridge between each of said poles, the narrowest portion of which bridge is from 2 to 4% of the diameter of said rotor, and a shaded portion of each pole of a width of from 20 to 30% of the entire width of said pole; and a field coil.

3. A four pole single phase induction motor of the shaded pole type, including: a rotor; a stator having poles of a width substantially 66% of the diameter of said rotor, an integral magnetic bridge between each of said poles, the narrowest portion of said bridge being substantially 3% oi' the diameter of said rotor, and a shaded portion of each pole of such a size that substantially 21% of the total ux in said pole passes through said shaded portion; and a field coil.

EARLE W. BALLENTINE. 

